A prior art of the present invention will be explained with reference to FIGS. 13 and 14. Reference numerals used in FIGS. 13 and 14 correspond to those used for explaining embodiments of the present invention.
A high pressure EGR apparatus 31 is known in the art as a technology for suppressing generation of nitrogen oxides (NOx) contained in exhaust gas of an engine 2. The high pressure EGR apparatus 31 will be explained with reference to FIG. 13.
The high pressure EGR apparatus 31 is generally called as an EGR apparatus, according to which a part of exhaust gas is re-circulated as EGR gas into an intake-air passage at a downstream side of a throttle valve 26 (that is, a high negative pressure portion). The EGR gas is mixed as un-combustible gas to intake air to suppress temperature increase in combustion chambers of the engine, so as to effectively suppress generation of NOx.
According to the high pressure EGR apparatus 31, a high pressure EGR control valve 33 is provided in a high pressure EGR passage 32 for re-circulating the EGR gas into an intake-air side, in order to control an opening degree of the high pressure EGR passage 32. The opening degree of the high pressure EGR control valve 33 is controlled by ECU (Engine Control Unit) so as to obtain such EGR amount (re-circulated amount of the exhaust gas per unit time) corresponding to an operational condition of the engine 2 (such as, engine rotational speed, engine load, and so on).
It is always required for the engine 2 to further reduce generation of NOx.
Recently, it is proposed, as disclosed in Japanese Patent Publication No. 2008-150955, to provide a low pressure EGR apparatus 1 in addition to the high pressure EGR apparatus 31 for the purpose of further reducing the generation of NOx. The low pressure EGR apparatus 1 will be explained with reference to FIG. 14.
The low pressure EGR apparatus 1 re-circulates a part of exhaust gas from a low pressure portion of an exhaust gas passage (a portion at which exhaust gas pressure is low, such as a downstream side of a DPF 29) to a low negative pressure portion of an intake-air passage 3 (a portion at which negative pressure for intake-air is low, such as an upstream side of a throttle valve 26). According to the low pressure EGR apparatus 1, a smaller amount of EGR gas is re-circulated to the engine 2.
More exactly, in the low pressure EGR apparatus 1 for a vehicle having a turbo-charger, a part of the exhaust gas on a downstream side of the exhaust gas passage 22 (that is, at a downstream side of an exhaust-gas turbine 28) is re-circulated to an upstream side of the intake-air passage 3 (that is, at an upstream side of a compressor 24). In other words, the exhaust gas in the low pressure portion of the exhaust gas passage 22 is re-circulated to the low negative pressure portion of the intake-air passage 3. As a result, it is possible that the smaller amount of EGR gas is re-circulated to the engine 2.
In an engine operating condition, for example, in an engine operating condition of high engine load, low density of EGR gas is required. It is, however, not possible to achieve such low density of EGR gas with the high pressure EGR apparatus 31. On the other hand, the low pressure EGR apparatus 1 can realize the low density of EGR gas so that generation of NOx can be more effectively suppressed in a wider engine operating range.
According to the low pressure EGR apparatus 1, a low pressure EGR control valve 5 is provided in a low pressure EGR passage 4 for re-circulating the EGR gas into the intake-air side, in order to control an opening degree of the low pressure EGR passage 4. In the same manner to the high pressure EGR control valve 33, the opening degree of the low, pressure EGR control valve 5 is controlled by the ECU so as to obtain such EGR amount corresponding to the operating condition of the engine 2 (such as, engine rotational speed, engine load, and so on).
As explained above, the low pressure EGR apparatus 1 re-circulates the exhaust gas in the low pressure portion of the exhaust gas passage 22 to the low negative pressure portion of the intake-air passage 3.
It is, therefore, on one hand possible for the low pressure EGR apparatus 1 to re-circulate small amount of the EGR gas into the engine 2. On the other hand, it is difficult for the low pressure EGR apparatus 1 to re-circulate large amount of the EGR gas into the engine 2. In other words, the low pressure EGR apparatus 1 can not meet a requirement for re-circulating the large amount of the EGR gas into the engine 2, even when there is such requirement in an engine operating condition.
According to a prior art, in view of the above problem, it is also proposed to provide an intake-air valve 6 (a negative pressure generating valve) at a portion of the intake-air passage 3, at which the EGR gas is re-circulated into the intake-air side by the low pressure EGR apparatus 1. In such an engine operating condition, in which large amount of EGR gas is required to be re-circulated to the engine 2, the intake-air valve 6 is operated in a valve closing direction so as to generate negative pressure. Namely, in the engine operating condition that re-circulation of the large amount of EGR gas by the low pressure EGR apparatus 1 is necessary, the negative pressure is generated by the intake-air valve 6 to re-circulate the large amount of EGR gas to the engine 2.
As already explained above, the opening degree of the low pressure EGR control valve 5 is controlled by the ECU depending on the engine rotational speed, the engine load, and so on.
On the other hand, the intake-air valve 6 is operated by the ECU to rotate in the valve closing direction only when the re-circulation of the large amount of EGR gas is required.
As above, the low pressure EGR control valve 5 and the intake-air valve 6 are respectively controlled depending on different operating conditions of the engine. Namely, the low pressure EGR control valve 5 and the intake-air valve 6 are independently operated from each other.
It is, therefore, necessary to provide not only an actuator J1 for driving the low pressure EGR control valve 5 but also another actuator J2 for driving the intake-air valve 6, which would cause increases in cost, in size, and in weight of the EGR apparatus.
Therefore, there is a demand to drive the low pressure EGR control valve 5 and the intake-air valve 6 by one actuator (for example, as disclosed in Japanese Patent Publications No. 2007-132305 or No. 2007-092664).
When the low pressure EGR control valve 5 and the intake-air valve 6 are simply driven by one actuator, both of the valves 5 and 6 are rotated together. It is not possible to obtain valve operating characteristics required for the respective valves.
Accordingly, according to the prior art, the actuator J1 for the low pressure EGR control valve 5 and the actuator J2 for the intake-air valve 6 are independently provided, even when such two actuators would cause increases in cost, in size, and in weight of the EGR apparatus.